The hydrocarbons emitted just after engine-start (cold-start HCs) cannot be decomposed by a conventional three-way-catalyst (TWC). This paper describes a cold-start HCs removal system that combines a three-way-catalyst with an HCs adsorbent (an HC-adsorption type TWC). The adsorbent is required not only to have high adsorption capacity to HCs, but also to hold the HCs until the TWC reaches its active temperature region. To meet this requirement, the geometrical relation between the adsorbent and TWC was important besides their respective properties. It was found that the multilayered structure of TWC/adsorbent/honeycomb support was most effective in promoting earlier TWC light-off. As for the adsorbent, the zeolite having two kinds of pore sizes (5.5--6.5-- and 7.5--8.5 ) was found to be supperior to the zeolites of a single pore size in adsorbing HCs of a wide range in carbon number. As for the TWC, the palladium-based supported catalyst loaded with cerium oxide was most effective in decomposing the HCs desorbed from the adsorbent. It was also found that the geometric surface area of honeycomb affected the HCs holding characteristics of the adsorbent and therefore the HCs conversion efficiency of the whole system. The HC-adsorption type TWC thus improved proved promoting performances for the removal of cold-start HCs.
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